1. Field of the Invention
This invention relates to a method of processing a mixture of bottom ash and fly ash by melting. When refuse of all kinds such as city garbage, sewer sludge and industrial wastes are incinerated, both bottom ash and fly ash are generated, the bottom ash being what remains inside the incinerator and the fly ash being what is collected mainly in the discharge gas processing system connected to the incinerator or in a dust collector provided in the discharge gas processing system. These ashes are thereafter subjected to a melting process inside a melting furnace such as an arc furnace, a plasma arc furnace, a resistance furnace, an induction furnace or a burner furnace in order to reduce their volume and to render them harmless. This invention relates to a method of processing such a mixture of bottom ash and fly ash by melting.
2. Prior Art
Bottom ash, which remains in the incinerator, is normally subjected to a moistening process in order to prevent it from flying around all over and to make it easier to handle. Examples of such a moistening process includes soaking the ash in water and spraying water over the ash. Whichever method may be used, bottom ash normally contains a significant amount of water (normally about 30-70 weight %) when taken out of the incinerator after such a moistening process.
According to prior methods of processing a mixture of bottom ash and fly ash by melting as disclosed, for example, in Japanese Patent Publication Tokkai 56-80613 and Japanese Utility Model Publication Jikkai 5-34426, the bottom ash containing a significant amount of water, as described above, is dried first to a certain extent. Both kinds of ash are thereafter stored in hoppers and are dropped into a space having a specified volume provided below the hoppers, and what has thus been dropped is fed into a melting furnace by means of a pusher, a screw conveyer or a rotary valve.
Such prior art methods, however, involve problems of the following kinds. Firstly, moistened ash still contains enough water even after it is dried to a certain extent, such that its adhesive force is quite strong and its angle of repose is substantially large. Such bottom ash is easily attached to the hopper or the conveyer and forms bridges and tunnels inside the hopper, frequently thereby clogging openings. Secondly, bottom ash containing a significant amount of water may generate sodium hydroxide from the sodium component also contained in the ash. If such bottom ash is mixed with fly ash, there is the danger that hydrogen gas may be generated by a reaction between sodium hydroxide and metallic aluminum or zinc contained in the fly ash. Thirdly, since this is basically a constant-volume method whereby bottom ash and fly ash are both dropped from their respective hoppers into a space having a specified volume, the weights of the ashes supplied into the melting furnace may vary sharply if there is a change in their bulk specific weights or their heights in the hoppers. This will cause instability in the operation of the melting furnace and also in the discharge of molten slag which is generated. Bottom ash and fly ash have significantly different properties and compositions, flying ash having a smaller specific weight and containing more volatile components than bottom ash. Thus, if there is a change in the supply rate or in the weight ratio between the two kinds of ash, although there may be no change in the supply rate, there results a change in the melting temperature as well as in pressure inside the melting furnace. If the operation of the melting furnace becomes unstable in this manner, the molten slag generated in the furnace may stagnate near the outlet or unmolten ash may fly out together with molten slag.